Location: Nimbus Arena, Cloud Kingdom
Phase 1: Bowser will attack with fire breath, tail swipes, and by throwing his wedding hat. Dodge his attacks. When he throws his hat, capture it with Cappy. Use the captured hat to attack Bowser. Spin the hat (shake Joy-Cons or rotate stick) to hit him multiple times. He'll be stunned.
Phase 2: He'll become more aggressive, with faster attacks. The strategy is the same: dodge, capture his hat, and hit him. After taking enough damage, he'll retreat. This is not a final defeat, but a temporary setback for Bowser.
Reward: Multi Moon